10/06/2026
Sometimes you’ve just gotta do the maths!
A customer of ours is on a mission to get his Sinclair C5 to do 40mph [we won’t ask why!]
The original motor was 250W and would push the C5 to about 15mph, so fitting a 750W motor should be a big improvement, right?
Well it turned out not quite so easy. The new motor briefly pushed the C5 up to 27mph but then expired in a cloud of smoke. Running the numbers on the gearing showed that the motor was only reaching 2600 rpm and not its 3300 peak efficiency speed. This meant it was not spinning fast enough to generate sufficient back emf and that the controller was forcing nearly 1kW into it with predictable results.
So how much power would be needed to reach the magic 40?
The power required is dominated by the aerodynamic drag which scales approximately with the cube of the speed.
We used the formula
P2 = P1(V2/V1)3
Substituting the values
P1 = 250W
V1 = 15mph
V2 = 40mph
It kicked out the answer of 4.7kW which means that he’ll need to either find a much more powerful motor, or do some serious work on improving the aero, and reducing the rolling resistance.
